During the drilling of oil in gas wells, a weighted drilling fluid or mud is circulated down the drill pipe under pressure, through an assembly of drill collars, through the jets or nozzles of a rotary drill bit, and back to the surface through the annulus between the pipe and the drill hole.
The specific weight of the drilling mud is adjusted and maintained such that the column of mud above any respective well formation exerts a hydrostatic pressure which is greater than the fluid pressure within such formation.
In situations when the drilling mud pressure is allowed to become less than a well formation pressure, then fluids from that formation will blow into the well annulus, forcing drilling mud up out of the well and further reducing the effective drilling mud hydrostatic pressure, until the well blows out from the formation pressure.
When the well "kicks" (when the formation pressure begins to overcome the drilling mud hydrostatic pressure) a heavier mud must be circulated into the well to create a greater hydrostatic pressure. The fluid circulation must be maintained, and frequently increased, during well control operations where the well is kicking or blowing out.
It is standard practice to use drill bits having small jet nozzles which limit and partially plug the circulating flow rates of the drilling mud through the drilling string. Occasionally, the mud may contain entrained loss circulation materials which are intended to clog the interstices of a formation where the fluid may be escaping. The loss circulation materials can sometimes tend to plug the jet nozzles.
When an increase in drilling mud circulation through the well is needed, an additional fluid circulation port through the wall of the drill collar may become very desirable. A chemical perforator of the present invention provides an improved approach to providing such an additional circulation port which overcomes many of the negative aspects of other approaches.
Drill collars have been perforated with shaped explosive charges. If the drill collars are eight inches or more in diameter, for example, the wall of the collar is about two inches thick and the shaped charges are not effective to produce a hole large enough for adequate drilling mud circulation.
Shaped charges or similar explosives also have been used to sever the drill collar above the drill bit. The mud circulation is then of course at a maximum, with the severed drill collar, to control the well. However, the portion of drill collar remaining in the hole and the drill bit must be fished out in a clean-up operation after the well is brought back under control, at a considerable expense in rig time.
Generally unsuccessful attempts have been made also to blow the nozzles out of the bits with "junk shots".
The chemical perforator of the present invention provides a clean and adequate mud circulating port hole with no subsequent fishing operation or other clean up problem.
Well conduit cut-off tools of the chemical type have been in use for several years. Such cutting tools are disclosed in U.S. Pat. Nos. 2,918,125 and 3,076,507. Specifically, the chemical perforator of the present invention incorporates a portion of the cut-off tool disclosed in commonly assigned and co-pending U.S. application Ser. No. 078,472, Filed Sept. 24, 1979, now U.S. Pat. No. 4,345,646. The disclosures of these patents and the pending application are hereby incorporated by reference as background information.